In mobile radiotelephones, it is known to make use of helically-shaped antennas that are usually mounted on the outside of the radiotelephone case. Such antennas can be of relatively small size, but they are placed outside the case in order to be associated with a ground plane which is itself placed inside the case of the radiotelephone.
The present trend in making radiotelephones is to eliminate any external antenna so that the antenna is located inside the case. The trend is also to reducing the size of the radiotelephone, or at least to integrating a larger number of components in a radiotelephone of given outside dimensions.
As a result it is advantageous in designing a radiotelephone to have available an internal antenna of dimensions that are relatively small.
In order to satisfy the first condition, proposals have been made to use patch type antennas in radiotelephones of the PiFa type or similar. Such patch antennas are essentially constituted by a ground plane and by a radiating plate, generally a radiating element that is substantially parallel to the ground plane, and also including a short circuit connection between the radiating element and the ground plane together with an antenna feed that is generally a 50 ohm (Ω) feed although that is not essential, and generally made in the form of a microstrip line, or in the form of coaxial connectors, or in the form of parallel-contact connectors having a characteristic impedance close 50Ω.
In the frequency band used for radiotelephones, and in particular in the frequency band corresponding to the general system for mobile communications (GSM), where the center frequency is about 920 megahertz (MHz), the minimum distance between the radiating element and the ground plane is about 7 millimeters (mm) to 10 mm, at least when the dielectric between the radiating element and the ground plane is air. This thickness of the order of 7 mm to 10 mm is considered as being too great for making radiotelephones. Unfortunately, it is found that if attempts are made to reduce the thickness of a PiFa antenna, e.g. in order to bring it down to less than 5 mm, then the passband of the antenna is considerably reduced, thereby making it practically unusable. Known patch antennas thus do not satisfy the second above-specified condition any better.